A power amplifier that is one of circuits commonly used in a high frequency integrated circuit (IC) for a wireless communication system is used to increase power of an applied signal. In detail, since signals having a same size and opposite polarities are applied to left and right ends of a basic transistor, and a difference between the signals is amplified by a power amplifier having a differential structure, the power amplifier is strong against noise in a semiconductor substrate, and power may be doubled by using the both ends of the basic transistor. However, when many transistors are connected for high output power, driving power for driving the many transistors is required.
FIGS. 1A and 1B are circuit diagrams of general power amplifiers using a differential structure. The general power amplifier of FIG. 1A is used to further effectively amplify output power, and includes transistors (hereinafter, referred to as basic transistors) 10 and 20 included in a power amplifier structure and additionally includes transistors (hereinafter, referred to as assistant transistors) 30 and 40 for assisting amplification of the basic transistors 10 and 20. The basic transistors 10 and 20 and the assistant transistors 30 and 40 share drains and sources, and gates of the assistant transistors 30 and 40 are connected to output ports 50 and 60 on the opposite side.
In the general power amplifier of FIG. 1A, since the assistant transistors 30 and 40 assist amplification, even when the basic transistors 10 and 20 have small sizes, higher power may be output, and required driving power may be decreased. However, the assistant transistors 30 and 40 operate according to power voltage Vs when a signal is not applied, and thus oscillation may be generated.
The general power amplifier of FIG. 1B is suggested to solve problems of the general power amplifier of FIG. 1A. The general power amplifier of FIG. 1B includes a transistor (hereinafter, referred to as a switch transistor) 70 for controlling oscillation and a switch transistor 80 having an opposite characteristic from the switch transistor 70 in addition to the components of the general amplifier of FIG. 1A. Here, a drain of the switch transistor 70 is connected to the sources of the basic transistors 10 and 20 and the sources of the assistant transistors 30 and 40, and a source of the switch transistor 70 is connected to a ground power source. Also, a source of the switch transistor 80 is connected to a power source 25 outputting a voltage having the same size as the power voltage Vs, and gates of the switch transistors 70 and 80 are connected to each other and applied with a signal for controlling oscillation.
In the general power amplifier of FIG. 1B, when a control signal applied to the switch transistor 70 is in a low level, the switch transistor 70 is unable to operate and thus a connection between a power amplifying stage and the ground power source is disconnected, whereas the switch transistor 80 is operated and thus a power amplifying stage and the power source 25 is connected. Also, since voltages of a power source 15 and the power source 25 are the same, a current cannot flow and thus oscillation of the assistant transistor 40 is blocked. On the other hand, when a control signal applied to the switch transistor is in a high level, the switch transistor 70 is operated and thus the power amplifying stage and the ground power source are connected, and the power amplifying stage may perform amplification. However, the switch transistor 70 has to bear all currents flowing through the basic transistors 10 and 20 and the assistant transistors 30 and 40, and a size of the switch transistor 70 has to be very big in order to prevent an amplification factor from decreasing.